1. Field of the Invention
The invention relates to methods and devices suitable for the production of spunbonded nonwoven fabrics.
2. Description of Related Art
The starting materials used are thermoplastic polymers that are melted and spun into fine spun filaments. The spun filaments are generally stretched aerodynamically and thereby obtain the desired strength. The filaments are deposited after the spinning process, or also after having first been placed on spools, on a deposit belt on which they come to lie on top of one another, forming the spunbonded nonwoven fabric.
The spinning process can also take place using the melt-blown process, in which the melt exiting from the spinnerets is entrained by an air stream at high pressure and high temperature, so that fibers with a low thickness are formed. These fibers can also be deposited to form a nonwoven fabric, and this is done primarily on deposit drums.
A process for the production of nonwoven fabrics is known from German Patent AS 1 303 556, in which the spun filaments are passed through a channel, aerodynamically stretched there, and subsequently deposited on a perforated, moving substrate, in the form of a nonwoven fabric. In order to ensure that the filaments are deposited in statistically random manner, a turbulence zone is provided below the air guide channel, which supports the filaments in being laid across one another. A very irregular pattern of the nonwoven fabric is obtained. A high degree of uniformity of the spunbonded nonwoven fabric is achieved in that several guide channels are provided, one after the other, and in that the filament sheets that exit from them are deposited in layers, on top of one another, to form the nonwoven fabric.
In order to be able to determine the desired uniformity of the nonwoven fabric and its strength in the lengthwise or crosswise direction, it is known from German Patent 39 07 215 A1 to structure the spinning manifolds as well as the filament take-off device so as to rotate. In this way, the disadvantages that occur in the so-called curtain method and that can lead to superimposition of individual filaments in certain regions are also supposed to be eliminated. In the curtain method, the nonwoven fabric has a preferential strength in the lengthwise direction, i.e. in the production direction, while the strength values in the crosswise direction are lower. This is supposed to be evened out by slanting the spinning manifolds along with the deposition and stretching device.
It is also known from German Patent 35 42 660 C2 to achieve deflection of the air stream, using a pivoting device arranged in parallel, below the take-off channel, in order to achieve a pendulum motion of the filaments in this way. The pivoting movement takes place in the running direction of the deposit belt in the production direction; among other things, so-called Coanda shells can be used, as they are described in German Patent 24 21 401 C3, for example. However, the measures provided are relatively sluggish, so that only slow oscillation of the filament sheet is possible. Uniform deposition is particularly difficult for fibers produced using the melt-blown method.